The present invention relates to an optical scanner in which the angle of a mirror fixed to a rotating shaft is detectaed and the rotating shaft is controlled to track a desired value indicating an instructed angle of the mirror, and a laser machining apparatus using the optical scanners.
In a laser machining apparatus for performing laser marking or perforating a printed board, coordinates of a machining position are transformed into coordinates to be used as mirror angle instruction data. Then, the angle of the mirror is changed by an optical scanner so that a work to be machined is irradiated at the machining position with a laser beam outputted from a laser oscillator. Ordinarily the laser beam has to be positioned two-dimensionally. Therefore, a pair of optical scanners corresponding to two orthogonal coordinate axes (X-axis and Y-axis) are used. In this case, the optical scanner for positioning the laser beam in the X-axis direction (hereinafter referred to as “X-axis scanner”) and the optical scanner for positioning the laser beam in the Y-axis direction (hereinafter referred to as “Y-axis scanner”) can be controlled independently of each other. However, it is necessary to synchronize the control of the two optical scanners with the emitting time of the laser beam.
Each optical scanner is provided with a sensor for detecting the angle of the mirror and a servo control circuit for performing feedback of the angle.
When the laser machining apparatus is used for perforating a printed board, there are two machining modes. One is a mode for machining a hole whose diameter is substantially as large as the diameter of the laser beam. The other is a mode for machining a hole whose diameter is larger than the diameter of the laser beam.
In the former case, the laser beam is emitted as pulses after the mirror is made to stand still at an angle corresponding to a machining position. Thus, a hole whose diameter is about 50–300 μm substantially equal to the laser beam diameter can be machined. The machining pitch in this machining mode is usually about 0.5–1 mm.
On the other hand, in the latter case, two sine waves 90° phase-shifted from each other are supplied as angle instructions to the X-axis scanner and the Y-axis scanner in the form of a target trajectory, while irradiation with a laser beam is repeated with a period such that beam spots overlap each other partially. The trajectory of the laser beam (hereinafter referred to as “beam trajectory”) follows a phase plane trajectory of the mirror's motion, that is, a circle whose diameter is proportional to the amplitude of the sine waves. Thus, when the aforementioned operation is repeated while the diameter of the beam trajectory is varied stepwise with its center being fixed, circular trepanning can be performed.
In addition, when the angle instructions to the X-axis scanner and the Y-axis scanner are varied, a desired beam trajectory can be drawn (hereinafter the machining mode for drawing a beam trajectory will be referred to as “target trajectory tracking machining”.).
Incidentally, U.S. Pat. No. 4,864,295 discloses a technique of a variable capacitance type angle sensor serving as means for detecting the rotating angle of a mirror. In the angle sensor, a dielectric flat plate attached to a rotating shaft is sandwiched between a pair of fixed pole plates so as to electrically detect the angle of the rotating shaft as a change of capacitance between the pole plates.
In addition, JP-A-4-127981 discloses a technique in which a mirror is irradiated with a laser beam for measuring the angle of the mirror, and reflected light of the laser beam is detected by a linear sensor so as to detect the rotating angle of the mirror.
The pulse oscillation frequency of a carbon dioxide laser is not lower than about 4 kHz, and the pulse oscillation frequency of a UV-YAG laser is not lower than about 20 kHz. On the other hand, the control frequency bandwidth of an optical scanner is about 1 kHz, which is lower than the pulse oscillation frequency of any laser. Therefore, if tracking control of the mirror can be made precisely with a frequency as high as possible, target trajectory tracking machining can be performed at a higher speed and with a higher accuracy.